Info Structure with Copper Pillar Having Reversed Profile

ABSTRACT

A method includes forming a first polymer layer to cover a metal pad of a wafer, and patterning the first polymer layer to form a first opening. A first sidewall of the first polymer layer exposed to the first opening has a first tilt angle where the first sidewall is in contact with the metal pad. The method further includes forming a metal pillar in the first opening, sawing the wafer to generate a device die, encapsulating the device die in an encapsulating material, performing a planarization to reveal the metal pillar, forming a second polymer layer over the encapsulating material and the device die, and patterning the second polymer layer to form a second opening. The metal pillar is exposed through the second opening. A second sidewall of the second polymer layer exposed to the second opening has a second tilt angle greater than the first tilt angle.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. application Ser. No.15/390,226, entitled “Info Structure with Copper Pillar having ReversedProfile,” filed Dec. 23, 2016, which claims the benefit of the followingprovisionally filed U.S. Patent application: Application Ser. No.62/395,528, filed Sep. 9, 2016 and entitled “Form Copper Pillar and InfoDie RDL with Reversed Shape;” which application is hereby incorporatedherein by reference.

BACKGROUND

With the evolving of semiconductor technologies, semiconductorchips/dies are becoming increasingly smaller. In the meantime, morefunctions need to be integrated into the semiconductor dies.Accordingly, the semiconductor dies need to have increasingly greaternumbers of I/O pads packed into smaller areas, and the density of theI/O pads rises over time. As a result, the packaging of the device diesbecomes more difficult, which adversely affects the yield of thepackaging.

To solve this problem, process have been developed to encapsulate adevice die in an encapsulating material such as molding compound, andforming redistribution lines to connect to the surface pads of thedevice die. The redistribution lines span to a greater area than thedevice die, and allow for more I/O pads to be formed without the need toincrease the area of the device die.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 24 illustrate the cross-sectional views of intermediatestages in the formation of packages in accordance with some embodiments.

FIGS. 25 and 26 illustrate the cross-sectional views of some portions ofa device die and a package, respectively, in accordance with someembodiments.

FIG. 27 illustrates a process flow of a packaging process in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A package and the method of forming the same are provided in accordancewith various exemplary embodiments. The intermediate stages of formingthe package are illustrated. The variations of the embodiments arediscussed. Throughout the various views and illustrative embodiments,like reference numbers are used to designate like elements.

FIGS. 1 through 24 illustrate the cross-sectional views of intermediatestages in the formation of an Integrated Fan-Out (InFO) package inaccordance with some embodiments. The steps shown in FIG. 1 through 24are also illustrated schematically in the process flow 300 as shown inFIG. 27.

FIG. 1 illustrates a cross-sectional view of wafer 10 in accordance withsome embodiments. Wafer 10 includes a plurality of semiconductor chips12. Wafer 10 further includes semiconductor substrate 14, which extendsinto semiconductor chips 12. Semiconductor substrate 14 may be a bulksilicon substrate or a silicon-on-insulator substrate. Semiconductorsubstrate 14 may also include other semiconductor materials includinggroup III, group IV, and group V elements. Integrated circuit 16 isformed at surface 14A of semiconductor substrate 14. Integrated circuit16 may include Complementary Metal-Oxide-Semiconductor (CMOS)transistors therein.

Semiconductor chips 12 may further include Inter-Layer Dielectric (ILD)17 over semiconductor substrate 14, and interconnect structure 22 overILD 17. Interconnect structure 22 includes dielectric layers 24, andmetal lines 20 and vias 18 formed in dielectric layers 24. In accordancewith some embodiments of the present disclosure, dielectric layers 24are formed of low-k dielectric materials. The dielectric constants (kvalues) of the low-k dielectric materials may be less than about 2.8, orless than about 2.5, for example. Metal lines 20 and vias 18 may beformed of copper, a copper alloy, or other metal-containing conductivematerials. Metal lines 20 and vias 18 may be formed using singledamascene and/or dual damascene processes.

Metal pads 26 are formed over interconnect structure 22, and may beelectrically coupled to circuit 16 through metal lines 20 and vias 18.Metal pads 26 may be aluminum pads or aluminum-copper pads, or mayinclude other metals. In accordance with some embodiments of the presentdisclosure, the metal features that are underlying and contacting metalpad 26 are metal lines. In accordance with alternative embodiments, themetal features that are underlying and contacting metal pads 26 aremetal vias.

Passivation layer 28 is formed to cover the edge portions of metal pads26. The central portion of each of metal pads 26 is exposed through anopening in passivation layer 28. Passivation layer 28 may be formed of anon-porous material. In accordance with some embodiments of the presentdisclosure, passivation layer 28 is a composite layer including asilicon oxide layer (not shown), and a silicon nitride layer (not shown)over the silicon oxide layer. In accordance with alternativeembodiments, passivation layer 28 is formed of Un-doped Silicate Glass(USG), silicon oxynitride, and/or the like. Although one passivationlayer 28 is shown, there may be more than one passivation layer.

Polymer layer 30 is coated over and covering passivation layer 28. Therespective step is illustrated as step 302 in the process flow shown inFIG. 27. Polymer layer 30 is formed of a polymer, which may be aphoto-sensitive polymer such as polyimide, polybenzoxazole (PBO),benzocyclobutene (BCB), or the like. A pre-baking is performed, followedby light exposure as shown in FIG. 1. Lithography mask 32, whichincludes opaque portions and transparent portions with the desirablepatterns, is used to light-expose polymer layer 30, wherein light 34penetrates through the transparent portions of lithography mask 32, andis blocked by the opaque portions of lithography mask 32.

Next, the light-exposed polymer layer 30 is developed, with someportions removed to form openings 31, and the center portions of theunderlying metal pads 26 are exposed to openings 31. The respective stepis illustrated as step 304 in the process flow shown in FIG. 27. Theresulting wafer 10 is shown in FIG. 2. In accordance with someembodiments of the present disclosure, the tilt angle γ of the sidewallsof polymer layer 30 is substantially vertical, for example, in the rangebetween about 85 degrees and about 95 degrees.

After the development of polymer layer 30, wafer 10 is further baked inorder to solidify polymer layer 30 and to drive solvents out. Therespective step is illustrated as step 306 in the process flow shown inFIG. 27. The resulting wafer 10 is shown in FIG. 3. The relatively hightemperature used for baking wafer 10 may be in the range between about370° C. and about 410° C. in accordance with some embodiments. Thebaking may last for a period of time between about 40 minutes and about120 minutes in accordance with some embodiments. The baking results inthe full solidification of polymer layer 30.

FIG. 25 illustrates a magnified view of a portion of wafer 10. As shownin FIG. 25, after the high-temperature baking, the sidewall profile ofpolymer layer 30 is smoothened and rounded, wherein sidewalls 30′ ofpolymer layer 30 have a smoothly and continuous rounded portion.Alternatively stated, from the bottom to the top of sidewall 30′, thetilt angles of sidewall 30′ may be continually and smoothly increased.Dashed lines 37 schematically represent the position and the profile ofthe sidewalls of polymer layer 30 before the high-temperature baking,and the rounded sidewalls 30′ illustrate the shape of polymer layer 30after the high-temperature baking. It is observed that thehigh-temperature baking causes polymer layer 30 to reflow toward thecenter line of metal pad 26. The reflow is a partial reflow, and polymerlayer 30 is softened with a high viscosity. The reflow of polymer layer30 causes the tilt angle of sidewalls 30′ to be reduced. For example, atthe locations where sidewall 30′ contacts metal pad 26 (or the immediateneighboring regions), the tilt angle γ′ of sidewall 30′ is in the rangebetween about 15 degrees and about 45 degrees, and may be in the rangebetween about 20 degrees and about 30 degrees. The reduced angle γ′ maycontribute to the reduction of stress between polymer layer 30 and metalpad 26 during the subsequent planarization, and the likelihood of thepeeling of polymer layer 30 from metal pad 26 is reduced.

The baking temperature is selected to be high enough to cause polymerlayer 30 to be slightly reflowed to generate the profile as shown inFIG. 25. The reflow, however, results in the width of opening 31 to bereduced from W1 to W2. In accordance with some embodiments, the widthdifference (W 1-W2) is in the range between about 6 μm and about 10 μm.Ratio W2/W1 may be in the range between about 0.8 and about 0.9. Tomaintain adequate contact area between metal pad 26 and the overlyingmetal pillar 46 (FIG. 26) that will be formed in subsequent steps, thereflow is controlled by selecting appropriate baking temperature. It isappreciated that the desirable baking temperature is partiallydetermined by the material of polymer 30. Furthermore, the composition(such as the amount of solvent that will be evaporated during thebaking) of polymer 30 also affects the desirable baking temperature, andhence experiments may be performed to result in the desirable reflow.

Next, referring to FIG. 4, Under-Bump Metallurgy (UBM) layer 36 isdeposited on polymer layer 30, for example, through physical VaporDeposition (PVD). In accordance with some embodiments of the presentdisclosure, UBM layer 36 is formed of a copper layer or a copper alloylayer. In accordance with alternative embodiments, UBM layer 36 includesa titanium layer and a seed layer that is formed of copper or a copperalloy. UBM layer 36 is also in contact with metal pads 26. Photo resist38 is then applied and patterned to form openings, through which UBMlayer 36 is exposed.

Referring to FIG. 5, metal regions 42 are selectively deposited intoopenings 40, for example, through plating. The respective step isillustrated as step 308 in the process flow shown in FIG. 27. Inaccordance with some exemplary embodiments, metal regions 42 are formedof a non-solder material that does not melt in reflow processes formelting solder. For example, metal regions 42 may be formed of copper ora copper alloy. Solder caps 44 may be formed on the top surfaces ofmetal regions 42, wherein solder caps 44 may be formed of a Sn—Ag alloy,a Sn—Cu alloy, a Sn—Ag—Cu alloy, or the like, and may be lead-freesolder caps or lead-containing solder caps. Solder caps 44 may also beformed through plating.

After the formation of metal regions 42 and solder caps 44, photo resist38 is removed, as shown in FIG. 6. Next, the portions of the UBM layer36 that are previously covered by photo resist 38 are removed, leavingmetal regions 42 and solder caps 44 un-removed, as shown in FIG. 7. Insubsequent discussion, the remaining portions of UBM layer 36 and metalregions 42 are in combination referred to as metal pillars 46.

In accordance with some embodiments, a reflow is performed so thatsolder caps 44 have rounded top surfaces. The solder in solder caps 44include some portions remaining overlapping metal regions 42, and may ormay not include some other portions flowing down to contact thesidewalls of metal pillars 46. The reflowed solder caps 44 may not coverthe bottom portions of the sidewalls of metal pillars 46. In accordancewith alternative embodiments, since solder caps 44 will be removed in asubsequent step, no reflow of solder caps 44 is performed.

Next, as shown in FIG. 8, a probing step is performed on solder caps 44to test the electrical properties of semiconductor chips 12. The probingis performed by putting probe pins 48 in contact with solder caps 44.Probe pins 48 are parts of probe card 50, which is electricallyconnected to a test equipment (not shown). Through the probing,defective semiconductor chips 12 are found, and good semiconductor chips12 are determined. Advantageously, solder caps 44 are softer than theunderlying metal pillars 46. Accordingly, the contact between probe pins48 and solder caps 44 is better than the contact between probe pins 48and metal pillars 46. Hence, the probing is more reliable than if soldercaps 44 are not formed.

After the probing, polymer layer 52 is formed to cover the top surfaceof wafer 10, as shown in FIG. 9. The respective step is illustrated asstep 310 in the process flow shown in FIG. 27. Hence, metal pillars 46and solder caps 44 are embedded in polymer layer 52, wherein the topsurface of polymer layer 52 is higher than the top ends of solder caps44. Polymer layer 52 may be formed of a material selected from the samecandidate materials (such as PBO) of polymer layer 30. A die-saw is thenperformed on wafer 10, and semiconductor chips 12 are separated fromeach other. The respective step is illustrated as step 312 in theprocess flow shown in FIG. 27. The separated semiconductor chips 12 arereferred to as device dies 12 hereinafter.

FIGS. 10 through 23 illustrate the packaging of device dies 12 to formInFO packages, so that the resulting electrical connectors (such assolder regions) of the InFO packages may be distributed to regionslarger than device dies 12. FIG. 10 illustrates carrier 54 and releaselayer 56 formed on carrier 54. Carrier 54 may be a glass carrier, andmay have a round top-view shape and a size of a common silicon wafer.Release layer 56 may be formed of a Light-To-Heat-Conversion (LTHC)coating material. The top surface of release layer 56 is planar.Dielectric layer 58 is formed on release layer 56. In accordance withsome embodiments, dielectric layer 58 is formed of a polymer, which mayalso be a photo-sensitive material such as polybenzoxazole (PBO),polyimide, benzocyclobutene (BCB), or the like, which may be patternedthrough light-exposure and development.

FIG. 11 through 13 illustrate the formation of metal posts 60. Therespective step is illustrated as step 313 in the process flow shown inFIG. 27. Throughout the description, metal posts 60 are alternativelyreferred to as through-vias 60 since metal posts 60 penetrate throughthe subsequently dispensed encapsulating material.

Referring to FIG. 11, metal seed layer 62 is formed, for example,through Physical Vapor Deposition (PVD). Metal seed layer 62 may includecopper, or may include a titanium layer and a copper layer over thetitanium layer in accordance with some embodiments. Photo resist 64 isformed over metal seed layer 62.

A light-exposure is then performed on photo resist 64 using a photolithography mask (not shown). After a subsequent development, openings66 are formed in photo resist 64, as shown in FIG. 11. Metal seed layer62 is exposed to openings 66. Openings 66 may have a sand-timer profile,with the bottom width W1 and top width W2 being greater than middlewidth W3. Furthermore, the smallest widths of openings 66 may be closeto the middle heights of openings 66.

The material of photo resist 64 is selected to make the resultingopenings 66 to have the sand-timer profile. In accordance with someexemplary embodiments, the photo resist includes TOK P50 series photoresist (manufactured by Tokyo Ohka Kogyo America Incorporated). The TOKP50 may include polyacrylate, cross-linker, and a photo-sensitiveinitiator in accordance with some embodiments. With the proper photoresist material being used, and process conditions for exposing anddevelopment being tuned, the sand-timer profile may be generated.

Next, as shown in FIG. 12, through-vias 60 are formed by plating. Theplating rate is controlled to ensure that the shape of the platedthrough-vias 60 follow the shape of openings 66. In subsequent steps,photo resist 64 is removed, and hence the underlying portions of metalseed layer 62 are exposed. The exposed portions of metal seed layer 62are then removed in an etching step. The resulting through-vias 60 areillustrated in FIG. 13. Throughout the description, the remainingportions of metal seed layer 62 are considered as parts of through-vias60, and are not illustrated separately.

Through-vias 60 have middle portions narrower than the respective topportions and the respective bottom portions. It is noted that FIG. 13illustrates the shapes of through-vias 60 in one vertical plane. Ifviewed from any other vertical plane, through-vias 60 may also have thesand-timer profile. The top-view shapes of through-vias 60 may becircles, rectangles, squares, hexagons, or the like.

FIG. 14 illustrates the perspective view of the placement of device dies12 onto the structure shown in FIG. 13, wherein device dies 12 arearranged as rows and columns. The good device dies 12 that are foundduring the probing are placed, and defective device dies 12 arediscarded. Through-vias 60 are not shown in FIG. 14, although they alsoexist.

FIG. 15 illustrates the cross-sectional view of a portion of thestructure shown in FIG. 14. In FIG. 15, only a single device die 12 andits surrounding through-vias 60 are illustrated. It is noted, however,the process steps shown in FIGS. 15 through 23 are performed at waferlevel, and are performed on all of device dies 12 on carrier 54. Devicedie 12 is placed on carrier 54, and is adhered to dielectric layer 58through Die-Attach Film (DAF) 68, which is an adhesive film. Therespective step is illustrated as step 314 in the process flow shown inFIG. 27.

Next, referring to FIG. 16, encapsulating material 70 is encapsulated ondevice die 12 and through-vias 60. The respective step is illustrated asstep 316 in the process flow shown in FIG. 27. Encapsulating material 70fills the gaps between neighboring through-vias 60 and the gaps betweenthrough-vias 60 and device die 12. Encapsulating material 70 may includea molding compound, a molding underfill, an epoxy, or a resin. Themolding compound may include a polymer (such as a resin) and fillers inthe polymer, wherein the filler may include the spherical particles ofsilica (amorphous SiO₂), aluminum oxide, etc. The top surface ofencapsulating material 70 is higher than the top ends of metal pillar 46and through-vias 60.

Next, as shown in FIG. 17, a planarization such as a Chemical MechanicalPolish (CMP) step or a grinding step is performed to thin encapsulatingmaterial 70 until through-vias 60 and metal pillars 46 are exposed. Therespective step is illustrated as step 318 in the process flow shown inFIG. 27. Due to the grinding, the top ends of through-vias 60 are level(coplanar) with the top surfaces of metal pillars 46, and are coplanarwith the top surface of encapsulating material 70. In the illustratedexemplary embodiments, the planarization is performed until metalpillars 46 are exposed. Accordingly, the portions of solder caps 44overlapping metal pillars 46 are removed. The portions of solder caps 44on the sidewalls of metal pillars 46 (if resulted by reflow) may remainafter the planarization.

FIG. 26 illustrates a cross-sectional view of a part of wafer 10including metal pillar 46. As shown in FIG. 26, polymer layer 30 isjoined to metal pad 26 with small tilt angle γ′. This may help releasethe stress applied on polymer layer 30 and metal pad 26 during theplanarization due to the elimination of sharp bottom angles of metalpillar 46. The small tilt angle γ′ is thus beneficial for reducing thedelamination between polymer layer 30 and metal pad 26, partly due tothe increased contact/overlap of polymer layer 30 over metal pad 26 uponPI reflow toward the center of pad 26, thus increasing the structuralintegrity at the joining interface and reducing the likelihood ofdelamination.

Metal pillar 46 includes lower portion 46A lower than the top surface ofpolymer layer 30, and upper portion 46B higher than the top surface ofpolymer layer 30. The thicknesses of lower portion 46A and upper portion46B are T1 and T2, respectively. In accordance with some embodiments,thickness ratio TI/T2 is in the range between about 1.1 and 1.4. Tiltangle β of the sidewall of portion 46B may be in the range between about60 degrees and about 105 degrees, or in the range between about 70degrees and about 90 degrees.

Further referring to FIG. 26, the curved portion of sidewalls 30′ mayhave radius R1, wherein ratio R1/T1 may be greater than about 0.2,greater than about 0.3, or in the range between about 0.3 and 0.5 inaccordance with some embodiments, the large radius R1 is more effectivein releasing stress in the subsequent planarization, as will bediscussed in subsequent paragraphs. Radius R1, however, cannot be toobig since the increased value R1 may result in the contact area betweenmetal pillar 46 and metal pad 26 to be too small.

Referring back to FIG. 17, after the planarization, through-vias 60 mayremain to have a sand-timer profile. Several profiles of through-vias 60may be resulted by the preceding process steps. In accordance with someembodiments of the present disclosure, a through-via 60 may include topportion 60A, middle portion 60B, and bottom portion 60C, wherein topportions 60A and bottom portion 60C may have vertical sidewalls anduniform widths (illustrated by dashed lines 60′), while the middleportion 60B has slanted sidewalls and continuously changed widths asillustrated. In accordance with alternatively embodiments, portions 60A,60B, and 60C all have gradually and continuously changed widths, with amiddle part of through-via 60 being narrowest, and the respective upperportions become increasingly wider, and lower portions becomeincreasingly wider also, as shown by solid lines in FIG. 17.

FIGS. 18 through 24 illustrate the formation of front-side RDLs andsolder regions. The respective step is illustrated as step 320 in theprocess flow shown in FIG. 27. Referring to FIG. 18, polymer layer 72 isformed, for example, using a photo-sensitive material. In accordancewith some embodiments, polymer layer 72 is formed of polyimide. Inaccordance with alternative embodiments, polymer layer 72 is formed ofother dielectric materials such as PBO. Openings 74 are formed inpolymer layer 72 to expose through-vias 60 and metal pillars 46.

The formation of polymer layer 72 and openings 74 includes dispensingpolymer layer 72, pre-baking polymer layer 72, performing alight-exposure on polymer layer 72, and developing the exposed polymerlayer 72. After the development, polymer layer 72 is baked. Inaccordance with some embodiments, openings 74 are narrower than openings31 (FIG. 2). Accordingly, it is desirable that the reflow effect ofpolymer layer 72 caused by the baking is less significant than polymerlayer 30, so that the width of openings 74 is not reduced as much as thereduction of openings 31. Otherwise, the contact area between theRedistribution Lines (RDLs) that will fill openings 74 will be reducedtoo much, and the contact resistance will be too high. Furthermore,since no CMP will be performed in the formation of the RDLs, it is lessdemanding in the requirement of stress reduction, and tilt angle al maybe greater than tilt angle γ′ (FIG. 25).

In accordance with some embodiments, to limit the reflow of polymerlayer 72, the baking temperature (performed after the development) islow, and is lower than the baking temperature of polymer layer 30. Inaccordance with some embodiments of the present disclosure, the bakingtemperature of polymer layer 72 is in the range between about 225° C.and about 275° C. The baking temperature of polymer layer 72 may also belower than the baking temperature of polymer layer 30 by a differencehigher than about 100° C., and the difference may also be in the rangebetween about 120° C. and 160° C. The baking period may be in the rangebetween about 40 minutes and about 80 minutes.

In accordance with some embodiments, polymer layer 30 and polymer layer72 are formed of a same material, for example, polyimide, and the bakingtemperature of polymer layer 30 is higher than the baking temperature ofpolymer layer 72 to induce more reflow in polymer layer 30 than polymerlayer 72. In accordance with alternative embodiments, polymer layer 30and polymer layer 72 are formed of different materials, for example,with one formed of polyimide and the other formed of PBO, and the bakingtemperature of polymer layer 30 is also higher than the bakingtemperature of polymer layer 72 to induce more reflow in polymer layer30 than polymer layer 72. In accordance with yet alternativeembodiments, polymer layer 30 and polymer layer 72 are formed ofdifferent materials. For example, polymer layer 30 may be formed of amaterial having a lower reflow temperature than polymer layer 72, andhence both layers 30 and 72 may be performed at a same temperature (orsimilar temperature with a difference smaller than about 20° C.), whilepolymer layer 30 still reflows more than polymer layer 72.

Since the lower baking temperature of polymer layer 72 results insmaller reflow effect than for polymer layer 30, after the baking ofpolymer layer 72, the originally vertical sidewalls of polymer layer 72is less tilted and less rounded than the sidewalls of polymer layer 30.In accordance with some embodiments of the present disclosure, tiltangle al is greater than angle γ′ (FIG. 26). The difference (β-γ′) maybe greater than about 30 degrees, and may be in the range between about30 degrees and about 60 degrees. Tilt angle α1 may be in the rangebetween about 70 degrees and 90 degrees. The lower baking temperature isalso beneficial for carrier 54, which cannot sustain very hightemperature.

Next, referring to FIG. 19, Redistribution Lines (RDLs) 80 are formed toconnect to metal pillars 46 and through-vias 60. RDLs 80 may alsointerconnect metal pillars 46 and through-vias 60. RDLs 80 include metaltraces (metal lines) over polymer layer 72 as well as vias extendinginto openings 74 (FIG. 18) to electrically connect to through-vias 60and metal pillars 46. In accordance with some embodiments, RDLs 80 areformed in a plating process, wherein each of RDLs 80 includes a seedlayer (not shown) and a plated metallic material over the seed layer.The seed layer and the plated material may be formed of the samematerial or different materials. RDLs 80 may include a metal or a metalalloy including aluminum, copper, tungsten, and/or alloys thereof. RDLs80 are formed of non-solder materials. The via portions of RDLs 80 maybe in physical contact with the top surfaces of metal pillars 46 andthrough-vias 60. In accordance with some embodiments, thickness ratioT1/T3 (with T1 shown in FIG. 26) is in the range between about 1.3 and1.6, wherein thickness T3 is the thickness of RDL 80.

Referring to FIG. 20, dielectric layer 82 is formed over RDLs 80 andpolymer layer 72. Dielectric layer 82 may be formed using a polymer,which may be selected from the same candidate materials as those ofpolymer layer 72. For example, dielectric layer 82 may comprisepolyimide, PBO, BCB, or the like. Openings 84 are also formed indielectric layer 82 to expose RDLs 80. The formation of openings 84 maybe performed through a photo lithography process. Polymer layer 82 maybe baked using processes similar to the baking of polymer layer 72, andhence tilt angle α2 of the sidewalls of polymer layer 82 may be in thesame range as tilt angle α1 of polymer layer 72.

FIG. 21 illustrates the formation of RDLs 86, which are electricallyconnected to RDLs 80. The formation of RDLs 86 may adopt similar methodsand materials to those for forming RDLs 80. RDLs 86 and 80 are alsoreferred to as front-side RDLs since they are located on the front sideof device die 12.

As shown in FIG. 22, an additional dielectric layer 88, which may be apolymer layer, is formed to cover RDLs 86 and dielectric layer 82.Dielectric layer 88 may be selected from the same candidate polymersused for forming dielectric layers 72 and 82. Opening(s) 90 are thenformed in dielectric layer 88 to expose the metal pad portions of RDLs86. In accordance with some embodiments, before forming dielectric layer88, one or a plurality of dielectric layers and RDL layers may be formedover and electrically coupling to RDLs 86, and the materials and methodsmay be similar to that of the underlying dielectric layers and RDLs.

FIG. 23 illustrates the formation of Under-Bump Metallurgies (UBMs) 92and electrical connectors 94 in accordance with some exemplaryembodiments. The formation of UBMs 92 may include deposition andpatterning. The formation of electrical connectors 94 may includeplacing solder balls on the exposed portions of UBMs 92 and thenreflowing the solder balls. In alternative embodiments, the formation ofelectrical connectors 94 includes performing a plating step to formsolder regions over RDLs 86 and then reflowing the solder regions.Electrical connectors 94 may also include metal pillars or metal pillarsand solder caps, which may also be formed through plating. Throughoutthe description, the combined structure including device die 12,through-vias 60, encapsulating material 70, and the corresponding RDLsand dielectric layers will be referred to as package 100, which may be acomposite wafer with a round top-view shape.

Next, package 100 is de-bonded from carrier 54. The respective step isillustrated as step 322 in the process flow shown in FIG. 27. Releaselayer 56 is also cleaned from package 100. The de-bonding may beperformed by projecting a light such as UV light or laser on releaselayer 56 to decompose release layer 56.

In the de-bonding, a tape (not shown) may be adhered onto dielectriclayer 88 and electrical connectors 94. In subsequent steps, carrier 54and release layer 56 are removed from package 100. A die-saw step isperformed to saw package 100 into a plurality of packages, eachincluding device die 12 and through-vias 60. One of the resultingpackages is shown as package 102 in FIG. 24.

FIG. 24 illustrates the bonding of package 102 with another package 200.The respective step is illustrated as step 324 in the process flow shownin FIG. 27. In accordance with some embodiments of the presentdisclosure, the bonding is performed through solder regions 98. Inaccordance with some embodiments, package 200 includes device dies 202,which may be memory dies such as Static Random Access Memory (SRAM)dies, Dynamic Random Access Memory (DRAM) dies, or the like. The memorydies may also be bonded to package substrate 204 in some exemplaryembodiments.

The embodiments of the present disclosure have some advantageousfeatures. By making the bottom portions of sidewalls of metal pillars tohave smaller tilt angles, the stress incurred in subsequentplanarization is reduced, and delamination is reduced or eliminated.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first polymer layer to cover a metal pad of a wafer,and patterning the first polymer layer to form a first opening. A firstsidewall of the first polymer layer exposed to the first opening has afirst tilt angle where the first sidewall is in contact with the metalpad. The method further includes forming a metal pillar in the firstopening, forming a dielectric layer encircling and covering the metalpillar, sawing the wafer to generate a device die, encapsulating thedevice die in an encapsulating material, performing a planarization toreveal the metal pillar, forming a second polymer layer over theencapsulating material and the device die, and patterning the secondpolymer layer to form a second opening. The metal pillar is exposedthrough the second opening. A second sidewall of the second polymerlayer exposed to the second opening has a second tilt angle greater thanthe first tilt angle.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first polymer layer to cover a metal pad of a wafer,and patterning the first polymer layer to form a first opening, with themetal pad exposed through the first opening. The method further includesbaking the wafer at a first temperature, forming a metal pillar in thefirst opening, forming a dielectric layer encircling and covering themetal pillar, sawing the wafer to generate a device die, encapsulatingthe device die in an encapsulating material, performing a planarizationto reveal the metal pillar, forming a second polymer layer over theencapsulating material and the device die, and patterning the secondpolymer layer to form a second opening, with the metal pillar exposedthrough the second opening. The second polymer layer is baked at asecond temperature lower than the first temperature. A redistributionline is formed to have a portion filling the second opening.

In accordance with some embodiments of the present disclosure, a packageincludes a device die, which includes a metal pad, a first polymer layercovering edge portions of the metal pad, and a metal pillar extendinginto the first polymer layer to contact a first sidewall of the firstpolymer layer. The first sidewall of the first polymer layer has a firsttilt angle. The package further includes an encapsulating materialencapsulating the device die. A top surface of the metal pillar iscoplanar with a top surface of the encapsulating material. A secondpolymer layer is over the encapsulating material and the device die. Aredistribution line has a portion extending into the second polymerlayer to contact a second sidewall of the second polymer layer. Thesecond sidewall has a second tilt angle greater than the first tiltangle.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a device die comprising: ametal pad; a first polymer layer covering edge portions of the metalpad; and a metal pillar extending into the first polymer layer tocontact a first sidewall of the first polymer layer, wherein the firstsidewall of the first polymer layer defines a first tilt angle withrespect to an upper surface of the metal pad; an encapsulating materialencapsulating the device die, wherein a top surface of the metal pillaris level with a top surface of the encapsulating material; a secondpolymer layer over the encapsulating material and the device die; and aredistribution line having a portion extending into the second polymerlayer to contact a second sidewall of the second polymer layer, whereinthe second sidewall has a second tilt angle greater than the first tiltangle.
 2. The package of claim 1, wherein the first tilt angle is in arange between about 15 degrees and about 45 degrees.
 3. The package ofclaim 1, wherein the first tilt angle is measured at a joint at whichthe first sidewall joins the upper surface of the metal pad.
 4. Thepackage of claim 1, wherein a lower portion of the first sidewall of thefirst polymer layer is curved with a radius, and a portion of the metalpillar in the first polymer layer has a thickness, wherein a ratio ofthe radius to the thickness is greater than about 0.2.
 5. The package ofclaim 1, wherein the first polymer layer and the second polymer layerare formed of a same polymer material.
 6. The package of claim 1,wherein the first polymer layer and the second polymer layer are formedof different polymer materials.
 7. The package of claim 1, wherein thesecond polymer layer comprises a bottom surface contacting a top surfaceof the metal pad and a top surface of the encapsulating material.
 8. Apackage comprising: a device die comprising: a metal pad; a firstdielectric layer having a portion overlapping a portion of the metalpad; and a metal pillar comprising a lower portion extending into thefirst dielectric layer to contact a top surface of the metal pad,wherein the lower portion of the metal pillar further contacts asidewall of the first dielectric layer to form an interface, and theinterface is curved; an encapsulating material encapsulating the devicedie; and a second dielectric layer over and contacting both theencapsulating material and the device die.
 9. The package of claim 8,wherein the interface joins the metal pad and defines a first tilt anglewith respect to the top surface of the metal pad, and the packagefurther comprises: a redistribution line having a portion extending intothe second dielectric layer to contact a second sidewall of the seconddielectric layer, wherein the second sidewall has a second tilt anglegreater than the first tilt angle.
 10. The package of claim 9, whereinthe first tilt angle is smaller than about 45 degrees.
 11. The packageof claim 9, wherein the metal pillar further comprises an upper portionover and connected to the lower portion, wherein the device die furthercomprises a third dielectric layer, with the upper portion of the metalpillar contacting a sidewall of the third dielectric layer to define anadditional interface, wherein the additional interface has a third tiltangle greater than the first tilt angle.
 12. The package of claim 8,wherein the interface has a radius, and a portion of the metal pillar inthe first dielectric layer has a thickness, and a ratio of the radius tothe thickness is greater than about 0.2.
 13. The package of claim 8,wherein the first dielectric layer and the second dielectric layer areformed of a same dielectric material.
 14. The package of claim 8,wherein the first dielectric layer and the second dielectric layer areformed of different dielectric materials.
 15. The package of claim 8,wherein the top surface of the metal pad and a top surface of the firstdielectric layer are substantially level with each other.
 16. Thepackage of claim 8, wherein from a bottom end of the interface to a topend of the interface, tilt angles of the interface increase gradually.17. A package comprising: a device die comprising: a metal pad; a firstpolymer layer having a portion overlapping a portion of the metal pad;and a conductive feature comprising: an upper portion over the firstpolymer layer; and a lower portion extending into the first polymerlayer to contact a top surface of the metal pad, wherein the lowerportion contacts a sidewall of the first polymer layer to form aninterface, and wherein from a bottom end to a top end of the interface,tilt angles of the interface increase.
 18. The package of claim 17,wherein the device die further comprises a passivation layer underlyingthe first polymer layer, wherein the passivation layer comprises abottom surface having a portion contacting the top surface of the metalpad.
 19. The package of claim 17, wherein at a point the interface joinsthe metal pad, the interface has a tilt angle with respect to the topsurface of the metal pad, and the tilt angle is smaller than about 45degrees.
 20. The package of claim 19, wherein the tilt angle is betweenabout 15 degrees and about 45 degrees.